With the increased use of plain paper copiers, development powders have enjoyed an increased popularity over liquid toners. Along with the increased use of development powders, magnetic brush units are becoming increasingly popular as opposed to cascading methods. Development powders used with magnetic brush units usually have an iron powder which serves as the carrier material. Inexpensive, untreated iron powders cannot be used in magnetic brush systems since such iron does not have sufficient stability toward rusting and has color and triboelectric charging properties adversely effected by variable humidity conditions. More specifically, the charge to mass ratio (C/M) of the carrier particles will decrease drastically upon exposure to high relative humidity. In order to solve this problem, those in the art have resorted to chemical plating and coating of the iron particles with polymers, oils, waxes and the like and have tried various treatments.
One method described in the literature for the treating of carrier particles is with perfluorinated carboxylic acid. Although this treatment has proven successful, the cost of such materials is relatively high and the number of solvents available for forming treating solutions is limited.
Another problem with prior art developer powders, which are employed in automatic copy machines, is carrier filming problems due to the mechanical rubbing of the carrier surface with the soft toner resins. The gradual accumulation of permanently attached film impairs the normal triboelectric charging of the toner particles in the toner mix. As a result, the toner is either less highly charged or sometimes oppositely charged giving rise to poor copy quality with a high degree of background.
In the literature, several types of plastic coating and electroplating of the carrier have been suggested to overcome the filming problems. Most of the prior art coating methods result in high cost and have other disadvantages such as yielding improper triboelectric charge properties and imparting a very high electrical resistance to the carrier that reduces its development electrode effect and results in poorly filled-in large image areas.